The present invention relates to carbon black for tire tread and more particularly to novel carbon black suitable for use as a reinforcement of rubber, such as tire tread of a passenger car, where high abrasion resistance and resilience are required. There are many kinds of carbon black for reinforcement of rubber having different characteristics. The characteristics of carbon black are a ma]or factor determining various properties of a rubber composition with which the carbon black is compounded.
For this reason, in compounding carbon black with a rubber composition, carbon black having characteristics suitable for applications of the rubber composition has been selected and used.
In recent years, in order to improve the fuel saving of automobiles, the development of a rubber composition which exhibits high abrasion resistance and excellent resilience when used as a tire tread has eagerly been desired.
In general, hard-grade carbon black having a small particle diameter and a large specific surface area has been used for rubber, such as tire tread, where it should exhibit high abrasion resistance under severe conditions.
On the other hand, in order to attain fuel saving through a decrease in the rolling resistance of the tire tread portion, it is necessary to increase the resilience of rubber by making use of carbon black having a large particle diameter and a small specific surface area or carbon black having a wide range of aggregate size distribution per given particle diameter.
Thus it was difficult to impart both of the high abrasion resistance and the high resilience to rubber at the same time, because these requirements are contradictory with each other from the viewpoint of the capability of carbon black in reinforcing rubber.
For this reason, various proposals have been made to impart both of the abrasion resistance and resilience to rubber through the compounding of the rubber with carbon black having particular properties, such as a particular specific surface area measured by the nitrogen adsorption method (hereinafter abbreviated to "N.sub.2 SA") and dibutyl phthalate (hereinafter abbreviated to "DBP") absorption, selected by evaluating various characteristics of carbon black in more detail on a microstructure level besides fundamental characteristics such as the particle diameter, specific surface area and structure of carbon black.
For example, a proposal has been made on carbon black belonging to hard carbon black having an N.sub.2 SA of at least 60 m.sup.2 /g and a DBP of at least 108 ml/100 g and satisfying the following characteristics requirements:
true specific gravity.ltoreq.1.8379-0.0006.times.N.sub.2 SA; PA0 tinting strength (%).gtoreq.0.6979.times.N.sub.2 SA-0.4278.times.DBP+203.3; and PA0 range of aggregate size distribution (hereinafter abbreviated to ".DELTA.Dst").gtoreq.30.6+0.6118.times.Dst (see U.S. Pat. No. 4,478,973 issued Oct. 23, 1984. PA0 true specific gravity.ltoreq.1.9080-0.0016 [N.sub.2 SA]; PA0 void diameter of particle aggregates (nm).gtoreq.62.2-0.236 [N.sub.2 SA]; and PA0 range of aggregate size distribution .DELTA.Dst.gtoreq.30.6+0.6118 [.DELTA.st].
According to this proposal, for example, the true specific gravity of a conventional carbon black is 1.7854 when the N.sub.2 SA is 93 m.sup.2 /g, while the true specific gravity of the proposed carbon black is as low as 1.7541 when the N.sub.2 SA is substantially the same as that of the conventional carbon black, i.e., 92 m.sup.2 /g.
Further, as is apparent from the above-described formulae, the proposed carbon black has the tinting strength and the range of aggregate size distribution each maintained above a given value.
On the other hand, Japanese Patent Application No. 62-154474 proposes a rubber composition having a high abrasion resistance imparted thereto by making use of carbon black having an N.sub.2 SA of at least 140 m.sup.2 /g and a relatively enlarged void size per given specific surface area of particle aggregates to improve the dispersion of the carbon black in the rubber component.
Despite the above-described proposals, the development of a rubber composition which has higher resilience while maintaining high abrasion resistance is still eagerly desired in the art.
Under the above-described circumstances, the present inventors have found that it is possible to effectively impart both of the above-described contradictory characteristics to rubber without the necessity of maintaining the tinting strength above a given value through setting of the true specific gravity per given specific surface area at a smaller value and the intra-aggregate pore diameter at a larger value with an N.sub.2 SA of 75 to 105 m.sup.2 /g, which has led to the completion of the present invention.
When the N.sub.2 SA is 75 to 105 m.sup.2 /g according to the present invention, the dispersion is better than that of the carbon black having an N.sub.2 SA of at least 140 mg/g described in the above-described Japanese Patent Application No. 62-154474. Accordingly, in the present invention, a definite preponderance of the abrasion resistance has been established by further increasing the void diameter of the particle aggregates per given N.sub.2 SA.